Autocapture pacing/sensing configuration

ABSTRACT

A cardiac electrical stimulation system that enhances the ability of the system to automatically detect whether an electrical stimulus results in heart capture or contraction. The cardiac electrical stimulation system may be utilized, for example, as a cardiac pacer or as a cardioverter defibrillator. The cardiac electrical stimulation system includes an electrical stimulation circuit that attenuates polarization voltages or “afterpotential” which develop at the heart tissue/electrode interface following the delivery of a stimulus to the heart tissue, which thereby allows the stimulation electrodes to be utilized to sense an evoked response to the electrical stimulus. The cardiac electrical stimulation system utilizes the stimulation electrodes to sense an evoked response, thereby eliminating the necessity for an indifferent electrode to sense an evoked response. The present invention allows accurate detection of an evoked response of the heart, to thereby determine whether each electrical stimulus results in capture.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a Continuation of U.S. application Ser. No.09/753,738, filed on Jan. 2, 2001, now issued as U.S. Pat. No.7,512,441, which is incorporated herein by reference in its entirety.U.S. application Ser. No. 09/753,738 is a Continuation-In-Part ofapplications Ser. No. 09/206,329, filed on Dec. 8, 1998, issued as U.S.Pat. No. 7,092,756, and entitled AUTOCAPTURE PACING/SENSINGCONFIGURATION and Ser. No. 09/206,896, filed on Dec. 8, 1998, issued asU.S. Pat. No. 6,169,921, and entitled AUTOCAPTURE DETERMINATION FOR ANIMPLANTABLE CARDIOVERTER DEFIBRILLATOR.

FIELD OF THE INVENTION

The present invention relates to cardiac rhythm management devices and,more particularly, to an apparatus and method that automatically detectswhether or not an electrical stimulus results in heart capture orcontraction.

BACKGROUND OF THE INVENTION

Cardiac pacers have enjoyed widespread use and popularity through timeas a means for supplanting some or all of an abnormal heart's naturalpacing functions. The various heart abnormalities remedied by pacemakersinclude total or partial heart block, arrhythmias, myocardialinfarctions, congestive heart failure, congenital heart disorders, andvarious other rhythm disturbances within the heart. The generalcomponents of a cardiac pacemaker include an electronic pulse generatorfor generating stimulus pulses to the heart coupled to an electrode leadarrangement (unipolar or bipolar) positioned adjacent or within apreselected heart chamber for delivering pacing stimulus pulses.

Regardless of the type of cardiac pacemaker employed to restore theheart's natural rhythm (i.e.: ventricular pacing, atrial pacing, or dualchamber pacing in both the atrium and ventricle), each type operates tostimulate excitable heart tissue cells adjacent to the electrode of thepacing lead employed with the pacemaker, which may or may not result incapture. Myocardial response to stimulation or “capture” is a functionof the positive and negative charges found in each myocardial cellwithin the heart. More specifically, the selective permeability of eachmyocardial cell works to retain potassium and exclude sodium such that,when the cell is at rest, the concentration of sodium ions outside ofthe cell membrane is significantly greater than the concentration ofsodium ions inside the cell membrane, while the concentration ofpotassium ions outside the cell membrane is significantly less than theconcentration of potassium ions inside the cell membrane. The selectivepermeability of each myocardial cell also retains other negativeparticles within the cell membrane such that the inside of the cellmembrane is negatively charged with respect to the outside when the cellis at rest. When a stimulus is applied to the cell membrane, theselective permeability of the cell membrane is disturbed and it can nolonger block the inflow of sodium ions from outside the cell membrane.The inflow of sodium ions at the stimulation site causes the adjacentportions of the cell membrane to lose its selective permeability,thereby causing a chain reaction across the cell membrane until the cellinterior is flooded with sodium ions. This process, referred to asdepolarization, causes the myocardial cell to have a net positive chargedue to the inflow of sodium ions. The electrical depolarization of thecell interior causes a mechanical contraction or shortening of themyofibril of the cell. The syncytial structure of the myocardiumtypically causes the depolarization originating in any one cell toradiate through the entire mass of the heart muscle so that all cellsare stimulated for effective pumping. Following heart contraction orsystole, the selective permeability of the cell membrane returns andsodium is pumped out until the cell is re-polarized with a negativecharge within the cell membrane. This causes the cell membrane to relaxand return to the fully extended state, referred to as diastole.

In a normal heart, the sino-atrial (SA) node initiates the myocardialstimulation of the atrium. The SA node comprises a bundle of uniquecells disposed within the roof of the right atrium. Each cell membraneof the SA node has a characteristic tendency to leak ions gradually overtime such that the cell membrane periodically breaks down and allows aninflow of sodium ions, thereby causing the SA node cells to depolarize.The SA node cells are in communication with the surrounding atrialmuscle cells such that the depolarization of the SA node cells causesthe adjacent atrial muscle cells to depolarize. This results in atrialsystole wherein the atria contract to empty blood into the ventricles.The atrial depolarization from the SA node is detected by theatrioventricular (AV) node which, in turn, communicates thedepolarization impulse into the ventricles via the Bundle of His andPurkinje fibers following a brief conduction delay. In this fashion,ventricular systole lags behind atrial systole such that the blood fromthe ventricles pumps through the body and lungs after being filled bythe atria. Atrial and ventricular diastole follow, wherein themyocardium is re-polarized and the heart muscle relaxed in preparationfor the next cardiac cycle. It is when this system fails or functionsabnormally that a cardiac pacer may be needed to deliver an electronicpacing stimulus for selectively depolarizing the myocardium of the heartso as to maintain proper heart rate and synchronization of the fillingand contraction of the atrial and ventricular chambers of the heart.

The success of a pacing stimulus in depolarizing or “capturing” theselected chamber of the heart hinges on whether the current of thepacing stimulus as delivered to the myocardium exceeds a thresholdvalue. This threshold value, referred to as the capture threshold, isrelated to the electrical field intensity required to alter thepermeability of the myocardial cells to thereby initiate celldepolarization. If the local electrical field associated with the pacingstimulus does not exceed the capture threshold, then the permeability ofthe myocardial cells will not be altered enough and thus nodepolarization will result. If, on the other hand, the local electricalfield associated with the pacing stimulus exceeds the capture threshold,then the permeability of the myocardial cells will be alteredsufficiently such that depolarization will result.

Changes in the capture threshold may be detected by monitoring theefficacy of stimulating pulses at a given energy level. If capture doesnot occur at a particular stimulation energy level which previously wasadequate to effect capture, then it can be surmised that the capturethreshold has increased and that the stimulation energy should beincreased. On the other hand, if capture occurs consistently at aparticular stimulation energy level over a relatively large number ofsuccessive stimulation cycles, then it is possible that the capturethreshold has decreased such that the stimulation energy is beingdelivered at level higher than necessary to effect capture.

The ability of a pacemaker to detect capture is desirable in thatdelivering stimulation pulses having energy far in excess of thepatient's capture threshold is wasteful of the pacemaker's limited powersupply. In order to minimize current drain on the power supply, it isdesirable to automatically adjust the pacemaker such that the amount ofstimulation energy delivered to the myocardium is maintained at thelowest level that will reliably capture the heart. To accomplish this, aprocess known as “capture verification” must be performed wherein thepacemaker monitors to determine whether an evoked depolarization occursin the preselected heart chamber following the delivery of each pacingstimulus pulse to the preselected chamber of the heart.

The conventional pacemaker typically includes a pacing output circuitdesigned to selectively generate and deliver stimulus pulses through alead to one or more electrodes positioned in the heart of a patient.While the conventional pacing circuit is generally effective indelivering stimulus pulses to a selected chamber of the heart, it hasbeen found that the detection of evoked depolarization or “captureverification” using the same electrode for pacing and sensing isdifficult due to polarization voltages or “afterpotentials” whichdevelop at the tissue/electrode interface following the application ofthe stimulation pulses. The ability to verify capture is furtheraffected by other variables including patient activity, body position,drugs being used, lead movement, noise etc.

Hence, a need exists for a cardiac pacing system having an autocapturepacing/sensing configuration that effectively avoids the affects ofafterpotentials or that attenuates polarization voltages or“afterpotentials” which develop at the heart tissue/electrode interfacefollowing the delivery of a stimulus to the heart tissue, and whichminimizes the number of required components of the cardiac pacingsystem.

The present invention meets these needs and provides additionalimprovements and advantages that will be recognized by those skilled inthe art upon review of the specification and figures.

SUMMARY OF THE INVENTION

The present invention provides an autocapture stimulation/sensingconfiguration for a cardiac electrical stimulation system. The systemmay be configured for sensing in either, both or between the atrium andventricle of the heart. The system may include either or both of anatrial lead and a ventricular lead. The atrial lead having one or moreatrial electrodes electrically and the ventricular lead having one ormore ventricular electrodes electrically. The atrial lead's electrodesmay include atrial tip electrodes and/or atrial ring electrodes. Theventricular lead's electrodes may include ventricular tip electrodes,ventricular superior vena cava electrodes, ventricular coil electrodes,and/or ventricular ring electrodes. The system may also include a leftventricular lead. The left ventricular lead may include one or more of aleft ventricular ring electrode, a left ventricular tip electrode, acoronary sinus ring electrode, a coronary sinus tip electrode and acoronary sinus coil. Further, the system may include an indifferentelectrode and/or a can electrode. The system also includes a pulsegenerator. The pulse generator is typically enclosed in a housing. Thepulse generator is electrically coupled to one or more of the atrialelectrodes and/or ventricular electrodes to provide an electricalstimulus to the atrium and/or ventricle of a heart. The system furtherincludes one or more sensing circuits at least one of the sensingcircuits configured to sense an evoked response. The evoked responsesensing circuit senses an evoked response to the electrical stimulusfrom the pulse generator. The evoked response sensing circuitelectrically is coupled to the atrial electrodes and/or the ventricularelectrodes to sense the evoked response. The evoked response sensingcircuit is typically configured to sense the evoked response between andthe pulse generator and is typically configured to provide an electricalstimulus between one or more of the atrial ring electrode to theindifferent electrode, the atrial ring electrode to the ventricular tipelectrode, the atrial ring electrode to the ventricular ring electrode,the atrial ring electrode to the can electrode, the atrial ringelectrode to the ventricular coil electrode, the atrial ring electrodeto the superior vena cava coil electrode, the atrial tip electrode tothe ventricular coil electrode, the atrial tip electrode to theventricular ring electrode, the atrial tip electrode to the ventriculartip electrode, the atrial tip electrode to the indifferent electrode,the atrial tip electrode to the can electrode, the atrial tip electrodeto the atrial ring electrode, the superior vena cava coil electrode tothe indifferent electrode, the superior vena cava coil electrode to thecan electrode, the superior vena cava coil electrode to the atrial tipelectrode, the superior vena cava coil electrode to the ventricular coilelectrode, the superior vena cava coil electrode to the ventricular tipelectrode, the ventricular coil electrode to the can electrode, theventricular coil electrode to the indifferent electrode, the ventricularring electrode to the indifferent electrode, the ventricular ringelectrode to the ventricular tip electrode, the ventricular tipelectrode to the indifferent electrode, the ventricular tip electrode tothe can electrode, the ventricular tip electrode to the ventricular coilelectrode, the superior vena cava coil electrode to the ventricular ringelectrode, the ventricular ring electrode to the can electrode, and theventricular ring electrode to the ventricular coil electrode.

A system in accordance with the present invention does not require anattenuation means if the pacing and sensing electrodes are independent,although the system may include an afterpotential attenuation means toattenuate afterpotentials. Afterpotentials result from the applicationof the pacing stimulus to the heart by said cardiac pacing system. Theafterpotential attenuation means is electrically coupled to thestimulation means. Suitable afterpotential attenuating means aredescribed in greater detail in co-pending applications Ser. No.09/070,158, filed Apr. 30, 1998, 09/088,864, filed Jun. 2, 1998, and08/977,272, filed Nov. 24, 1997, each of which have been assigned to thesame assignee as the present application, the entire disclosures ofwhich are incorporated herein by reference for any purpose.

The method in accordance with the present invention automaticallydetermines whether or not an electrical stimulus evokes a response inthe heart. The method utilizes a cardiac electrical stimulation systemto apply the electrical stimulus. The system typically includes a pulsegenerator and an evoked response sensing circuit attached to atrialand/or ventricular leads configured as described above. An electricalstimulus is provided to at least one of an atrium or ventricle of aheart. A signal indicative of the evoked response by the heart to theelectrical stimulus is then sensed. The signal associated with an evokedresponse is typically sensed between at least one of the atrialelectrodes and the ventricular electrodes.

The present invention may be utilized with unipolar or bipolar atrialand ventricular pacing and sensing leads, and which may attenuate andshorten afterpotentials and thereby enhance the detection of an evokedresponse in a preselected chamber of the heart. When used with atrialautocapture verification or evoked response detection, a bipolar atriallead is preferred and the ventricular lead may be either unipolar orbipolar. Likewise, when used with ventricular autocapture, theventricular lead is preferably bipolar and the atrial lead can be eitherunipolar or bipolar. The present invention may utilize the pacingelectrodes of a bipolar atrial lead and bipolar ventricular lead to bothpace and sense an evoked response in a preselected chamber of the heart.The pacing system of the present invention may reduce the requiredblanking period and attenuate afterpotential developed at the pacingelectrodes.

These and other objects and advantages of the present invention will bereadily apparent to those skilled in the art from a review of thefollowing detailed description of the preferred embodiment inconjunction with the accompanying claims and drawings in which likenumerals in the several views refer to corresponding parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cardiac pacing system in accordance with thepresent invention;

FIG. 2 illustrates a cardiac pacing/defibrillation system in accordancewith the present invention.

FIG. 3 is a schematic diagram of a portion of the cardiac pacingsystem's pacing/sensing circuitry in accordance with the presentinvention;

FIG. 4 is a schematic diagram of an alternate embodiment of a portion ofthe cardiac pacing system's pacing/sensing circuitry in accordance withthe present invention;

FIG. 5 is a schematic diagram of an alternate embodiment of a portion ofthe cardiac pacing system's pacing/sensing circuitry in accordance withthe present invention;

FIG. 6 is a schematic diagram of an alternate embodiment of a portion ofthe cardiac pacing system's pacing/sensing circuitry in accordance withthe present invention;

FIG. 7 is a schematic diagram of an alternate embodiment of a portion ofthe cardiac pacing system's pacing/sensing circuit of the presentinvention;

FIG. 8 is a schematic diagram of an alternate embodiment of the pacingoutput circuit of the present invention;

FIG. 9 depicts a resulting pacing waveform observable between the ringand tip of a pacing lead positioned within the heart of a patient, whenutilizing a conventional pacing circuit;

FIG. 10 depicts a resulting pacing waveform observable between the ringand tip of a pacing lead positioned within the heart of a patient, whenutilizing the afterpotential attenuation means of the present invention;

FIG. 11 depicts waveforms resulting from an atrial pacing stimulus and aventricular pacing stimulus, wherein a first waveform is sensed with theatrial ring electrode and atrial tip electrode of the atrial pacing leadand a second waveform shown for comparison is sensed with a surface ECG,while utilizing a conventional coupling capacitor, and wherein thepacing output or stimulus is below the required threshold output;

FIG. 12 depicts waveforms resulting from an atrial pacing output orstimulus, wherein the first waveform is sensed with the atrial ringelectrode and atrial tip electrode of the atrial pacing lead and asecond waveform shown for comparison is sensed with a surface ECG, whileutilizing a conventional coupling capacitor, and wherein the pacingoutput is above the required threshold output;

FIG. 13 depicts waveforms resulting from an atrial pacing output and aventricular pacing output, wherein the first waveform is sensed with theatrial ring electrode and atrial tip electrode of the atrial pacing leadand a second waveform shown for comparison is sensed with a surface ECG,while utilizing the afterpotential attenuation means of the presentinvention, and wherein the pacing output is below the required thresholdoutput;

FIG. 14 depicts waveforms resulting from an atrial pacing output and aventricular pacing output, wherein the first waveform is sensed with theatrial ring electrode and atrial tip electrode of the atrial pacing leadand a second waveform shown for comparison is sensed with a surface ECG,while utilizing the afterpotential attenuation means of the presentinvention, and wherein the pacing output is above the required thresholdoutput;

FIG. 15 depicts waveforms resulting from an atrial pacing output,wherein the first waveform is sensed with the atrial ring electrode andan indifferent electrode, and a second waveform shown for comparison issensed with a surface ECG, while utilizing the afterpotentialattenuation means of the present invention, and wherein the pacingoutput is below the required threshold output;

FIG. 16 depicts waveforms resulting from an atrial pacing output,wherein the first waveform is sensed with the atrial ring electrode andan indifferent electrode, and a second waveform shown for comparison issensed with a surface ECG, while utilizing the afterpotentialattenuation means of the present invention, and wherein the pacingoutput is above the required threshold output;

FIG. 17 depicts waveforms resulting from an atrial pacing output,wherein the first waveform is sensed with the atrial ring electrode andventricular tip electrode, and a second waveform shown for comparison issensed with a surface ECG, while utilizing the afterpotentialattenuation means of the present invention, and wherein the pacingoutput is below the required threshold output;

FIG. 18 depicts waveforms resulting from an atrial pacing output,wherein the first waveform is sensed with the atrial ring electrode andventricular tip electrode, and a second waveform shown for comparison issensed with a surface ECG, while utilizing the afterpotentialattenuation means of the present invention, and wherein the pacingoutput is above the required threshold output and;

FIG. 19 depicts waveforms resulting from an atrial pacing stimulus and aventricular pacing stimulus, wherein a first waveform is sensed with asuperior vena cava ventricular lead electrode and an indifferentelectrode, the second or lower waveform is sensed with a ventricularcoil electrode and an atrial ring electrode, and the third or upperwaveform, shown for comparison, is sensed with a surface ECG, whileutilizing the afterpotential attenuation means of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a cardiac pacing system in accordance with thepresent invention. The cardiac pacing system includes a cardiac pacer10, an atrial lead 12 and a right ventricular lead 14. In addition or asan alternative to right ventricular lead 14, the cardiac pacing systemmay include a left ventricular lead 80. The cardiac pacer 10 includes aheader 16 and can 18, wherein a pulse generator 20 including pacing andsensing circuits 22 are contained therein. An indifferent electrode 24of suitable known construction is positioned on the can 18 such that theindifferent electrode 24 is electrically isolated from the can 18 and iselectrically coupled to the sensing circuit 22. Atrial lead 12, rightventricular lead 14 and left ventricular lead 80 are engaged to header16 and may be electrically coupled to the pulse generator 20 and pacingand sensing circuits 22 in a known suitable fashion. The atrial lead 12is positioned in the atrium of the heart 26, wherein the atrial lead 12includes a tip electrode 28 and ring electrode 30. The right ventricularlead 14 is positioned within the right ventricle of the heart 26,wherein the ventricular lead 14 includes a tip electrode 32 and ringelectrode 34. The left ventricular lead 80 is positioned within the leftventricle of the heart 26, wherein the left ventricular lead 80 includesa tip electrode 84 and ring electrode 82.

FIG. 2 illustrates a cardiac pacing/defibrillating system in accordancewith the present invention. The cardiac pacing/defibrillating systemincludes an implantable cardioverter defibrillator 11, atrial lead 12and ventricular lead 14. In addition or alternatively, the pacingdefibrillation system may include a coronary sinus lead. The implantablecardioverter defibrillator 11 includes a header 16 and can 18, wherein apulse generator 20 including pacing and sensing circuits 22 anddefibrillator circuit 24 are contained therein. An indifferent electrode26 of suitable known construction is positioned on the can 18 such thatthe indifferent electrode 24 is electrically isolated from the can 18and is electrically coupled to the sensing circuit 22. Atrial lead 12,ventricular lead 14 and coronary sinus lead 15 are engaged to header 16and may be electrically coupled to the pulse generator 20, pacing andsensing circuit 22, and defibrillator circuit 23 in a known suitablefashion. The atrial lead 12 is positioned in the atrium of the heart 26,wherein the atrial lead 12 includes a tip electrode 28 and ringelectrode 30. The ventricular lead 14 is positioned through the superiorvena cava with the distal portion positioned within the ventricle of theheart 26. The ventricular lead 14 may include a tip electrode 32 andventricular coil electrode 33 and superior vena cava coil electrode 35.Those skilled in the art will appreciate that the ventricular lead 14may be utilized both for ventricular pacing and as a defibrillator lead,wherein the cardioverter defibrillator 11 is of the conventional typehaving modification to the pacing sensing circuit 22 as described belowin greater detail. The coronary sinus lead 15 is positioned through thecoronary sinus with the distal portion positioned within the coronaryartery of heart 26. The coronary sinus lead 15 may include a coronarysinus tip electrode 17, a coronary sinus ring electrode 19 and acoronary sinus coil 21.

FIG. 3 illustrates a portion of the pacing and sensing circuit 22combining the electrode elements of cardiac pacer 10 and implantablecardioverter defibrillator 11. The circuit 22 includes an atrialintrinsic sense amplifier 36 electrically coupled between the atrialring 30 and atrial tip 28. The circuit 22 also includes a ventricularintrinsic sense amplifier 38 electrically coupled between theventricular ring electrode 34 and the ventricular tip electrode 32. Aseparate evoked response sense amplifier 40 is shown electricallycoupled to a multi-switch 42, wherein the evoked response senseamplifier 40 may be electrically coupled to sense evoked responsewaveforms resulting from either an atrial pacing stimulus or ventricularpacing stimulus with any of the following sensing configurations: theatrial ring electrode to the indifferent electrode, the atrial ringelectrode to the ventricular tip electrode, the atrial ring electrode tothe ventricular ring electrode, the atrial ring electrode to the can,the atrial ring electrode to the ventricular coil, the atrial ringelectrode to the superior vena cava coil, the atrial tip electrode tothe ventricular coil, the atrial tip electrode to the ventricular ringelectrode, the atrial tip electrode to the ventricular tip electrode,the atrial tip electrode to the indifferent electrode, the atrial tipelectrode to the can, the atrial tip electrode to the atrial ringelectrode, the superior vena cava coil to the indifferent electrode, thesuperior vena cava coil to the can, the superior vena cava coil to theatrial tip electrode, the superior vena cava coil to the ventricularcoil electrode, the superior vena cava coil to the ventricular tipelectrode, the ventricular coil to the can, the ventricular coil to theindifferent electrode, the ventricular ring electrode to the indifferentelectrode, the ventricular ring electrode to the ventricular tipelectrode, the ventricular tip electrode to the indifferent electrode,the ventricular tip electrode to the can, the ventricular tip electrodeto the ventricular coil, the superior vena cava coil to the ventricularring electrode, the ventricular ring electrode to the can, and theventricular ring electrode to the ventricular coil. Those skilled in theart will appreciate that the preferred sensing configuration utilizingthe separate evoked response sense amplifier 40 will vary depending uponwhether the pacing stimulus is unipolar or bipolar and whether thepacing stimulus is directed in the atrium or ventricle. Those skilled inthe art will appreciate that the sensing configuration may utilize asecond evoked response amplifier 40. The use of a second evoked responseamplifier 40 typically depends on whether the pacing stimulus isunipolar or bipolar and whether the pacing stimulus is directed to theatrium or ventricle.

When unipolar pacing in the ventricle, the ventricle evoked response maybe sensed without the need for an attenuation means by sensing betweenthe atrial ring electrode to the atrial tip electrode, the superior venacava coil to the atrial tip electrode, the superior vena cava coil tothe atrial ring electrode, the ventricular ring electrode to the atrialring electrode, the ventricular ring electrode to the superior vena cavacoil, the right ventricular coil to the atrial tip electrode, the rightventricular coil to the atrial ring electrode, the right ventricularcoil to the superior vena cava coil, the indifferent electrode to theatrial tip electrode, the indifferent electrode to the atrial ringelectrode, the indifferent electrode to the superior vena cava coil, andthe indifferent electrode to the right ventricular coil. Alternatively,the ventricular evoked response from unipolar pacing may be sensed withan attenuation means by sensing between the ventricular tip electrode tothe atrial tip electrode, the ventricular tip electrode to the atrialring electrode, the ventricular tip electrode to the superior vena cavacoil, the right ventricular coil to the ventricular tip electrode, thecan to the atrial tip electrode, the can to the atrial ring electrode,the can to the superior vena cava coil, the can to the ventricular tipelectrode, the can to the ventricular ring electrode, the can to theright ventricular coil, and the indifferent electrode to the can.

In addition, when unipolar pacing in the left ventricle, the leftventricular evoked response may also be sensed without the need for anattenuation means by sensing between the atrial tip electrode to theleft ventricular ring electrode, the atrial ring electrode to the leftventricular ring electrode, the superior vena cava coil to the leftventricular ring electrode, the right ventricular tip electrode to theleft ventricular ring electrode, the right ventricular ring electrode tothe left ventricular ring electrode, the right ventricular coil to theleft ventricular ring electrode, the can to the left ventricular ringelectrode, the indifferent electrode to the left ventricular ringelectrode, the atrial tip electrode to the coronary sinus ringelectrode, the atrial ring electrode to the coronary sinus ringelectrode, the superior vena cava coil to the coronary sinus ringelectrode, the right ventricular tip electrode to the coronary sinusring electrode, the right ventricular ring electrode to the coronarysinus ring electrode, the right ventricular coil to the coronary sinusring electrode, the can to the coronary sinus ring electrode, theindifferent electrode to the coronary sinus ring electrode, the leftventricular ring electrode to the coronary sinus ring electrode, theatrial tip electrode to the coronary sinus tip electrode, the atrialring electrode to the coronary sinus tip electrode, the superior venacava coil to the coronary sinus tip electrode, the right ventricular tipelectrode to the coronary sinus tip electrode, the right ventricularring electrode to the coronary sinus tip electrode, the rightventricular coil to the coronary sinus tip electrode, the can to thecoronary sinus tip electrode, the indifferent electrode to the coronarysinus tip electrode, the left ventricular ring electrode to the coronarysinus tip electrode, the coronary sinus ring electrode to the coronarysinus tip electrode, the atrial tip electrode to the coronary sinuscoil, the atrial ring electrode to the coronary sinus coil, the superiorvena cava coil to the coronary sinus coil, the right ventricular tipelectrode to the coronary sinus coil, the right ventricular ringelectrode to the coronary sinus coil, the right ventricular coil to thecoronary sinus coil, the can to coronary sinus coil, the indifferentelectrode to the coronary sinus coil, the left ventricular ringelectrode to the coronary sinus coil, the coronary sinus ring electrodeto the coronary sinus coil, and the coronary sinus tip electrode to thecoronary sinus coil. Alternatively, the left ventricular evoked responsefrom unipolar pacing may also be sensed with an attenuation means bysensing between the atrial tip electrode to the left ventricular tipelectrode, the atrial ring electrode to the left ventricular tipelectrode, the superior vena cava coil to the left ventricular tipelectrode, the right ventricular tip electrode to the left ventriculartip electrode, the right ventricular ring electrode to the leftventricular tip electrode, the right ventricular coil to the leftventricular tip electrode, the can to the left ventricular tipelectrode, the indifferent electrode to the left ventricular tipelectrode, the left ventricular ring electrode to the left ventriculartip electrode, the left ventricular tip electrode to the coronary sinusring electrode, the left ventricular tip electrode to the coronary sinustip electrode and the left ventricular tip electrode to the coronarysinus coil.

When bipolar pacing in the ventricle, the ventricular evoked responsemay be sensed without the need for an attenuation means by sensingbetween the atrial ring electrode to the atrial tip electrode, thesuperior vena cava coil to the atrial tip electrode, the superior venacava coil to the atrial ring electrode, the can to the atrial tipelectrode, the can to the atrial ring electrode, the can to the superiorvena cava coil, the indifferent electrode to the atrial tip electrode,the indifferent electrode to the atrial ring electrode, and theindifferent electrode to the superior vena cava coil. Alternatively, theventricular evoked response from bipolar pacing may be sensed with anattenuation means by sensing between the ventricular tip electrode tothe atrial tip electrode, the ventricular tip electrode to the atrialring electrode, the ventricular tip electrode to the superior vena cavacoil, the ventricular ring electrode to the ventricular tip electrode,the right ventricular coil to the ventricular tip electrode, thesuperior vena cava coil to the ventricular ring electrode, the superiorvena cava coil to the right ventricular coil, the atrial ring electrodeto the ventricular ring electrode, the atrial ring electrode to theright ventricular coil, the atrial tip electrode to the ventricular ringelectrode, the atrial tip electrode to the right ventricular coil, thecan to the ventricular tip electrode, the can to the ventricular ringelectrode, the can to the right ventricular coil, the indifferentelectrode to the ventricular tip electrode, the indifferent electrode tothe ventricular ring electrode, the indifferent electrode to the rightventricular coil, and the indifferent electrode to the can.

In addition, when bipolar pacing in the left ventricle, the leftventricular evoked response may also be sensed without the need for anattenuation means by sensing between the atrial tip electrode to thecoronary sinus ring electrode, the atrial tip electrode to the coronarysinus tip electrode, the atrial tip electrode to the coronary sinuscoil, the atrial ring electrode to the coronary sinus ring electrode,the atrial ring electrode to the coronary sinus tip electrode, theatrial ring electrode to the coronary sinus coil, the superior vena cavacoil or the coronary sinus ring electrode, the superior vena cava coilto the coronary sinus tip electrode, the superior vena cava coil to thecoronary sinus coil, the can to the coronary sinus ring electrode, thecan to the coronary sinus tip electrode, the can to the coronary sinuscoil, the indifferent electrode to the coronary sinus ring electrode,the indifferent electrode to the coronary sinus tip electrode, theindifferent electrode to the coronary sinus coil, the coronary sinusring electrode to the coronary sinus tip electrode, the coronary sinusring electrode to the coronary sinus coil, and the coronary sinus tipelectrode to the coronary sinus coil. Alternatively, the leftventricular evoked response from bipolar pacing may also be sensed withan attenuation means by sensing between the atrial tip electrode to theleft ventricular ring electrode, the atrial tip electrode to the leftventricular tip electrode, the atrial ring electrode to the leftventricular ring electrode, the atrial ring electrode to the leftventricular tip electrode, the superior vena cava coil to the leftventricular ring electrode, the superior vena cava coil to the leftventricular tip electrode, the right ventricular tip electrode to theleft ventricular ring electrode, the right ventricular tip electrode tothe left ventricular tip electrode, the right ventricular tip electrodeto the coronary sinus ring electrode, the right ventricular tipelectrode to the coronary sinus tip electrode, the right ventricular tipelectrode to the coronary sinus coil, the right ventricular ringelectrode to the left ventricular ring electrode, the right ventricularring electrode to the left ventricular tip electrode, the rightventricular ring electrode to the coronary sinus ring electrode, theright ventricular ring electrode to the coronary sinus tip electrode,the right ventricular ring electrode to the coronary sinus coil, theright ventricular coil to the left ventricular ring electrode, the rightventricular coil to the left ventricular tip electrode, the rightventricular coil to the coronary sinus ring electrode, the rightventricular coil to the coronary sinus tip electrode, the rightventricular coil to the coronary sinus coil, the can to the leftventricular ring electrode, the can to the left ventricular tipelectrode, the indifferent electrode to the left ventricular ringelectrode, the indifferent electrode to the left ventricular tipelectrode, the left ventricular ring electrode to the left ventriculartip electrode, the left ventricular ring electrode to the coronary sinusring electrode, the left ventricular ring electrode to the coronarysinus tip electrode, the left ventricular ring electrode to the coronarysinus coil, the left ventricular tip electrode to the coronary sinusring electrode, the left ventricular tip electrode to the coronary sinustip electrode, and the left ventricular tip electrode to the coronarysinus coil.

When unipolar pacing in the atrium, the atrial evoked response may besensed without the need for an attenuation means by sensing between theatrial ring electrode to the superior vena cava coil, the atrial ringelectrode to the ventricular tip electrode, the superior vena cava coilto the ventricular tip electrode, the atrial ring electrode to theventricular ring electrode, the atrial ring electrode to the rightventricular coil, the superior vena cava coil to the ventricular ringelectrode, the superior vena cava coil to the right ventricular coil,the ventricular tip electrode to the ventricular ring electrode, theventricular tip electrode to the right ventricular coil, the superiorvena cava coil to the indifferent electrode, the ventricular tipelectrode to the indifferent electrode, the ventricular ring electrodeto the indifferent electrode, and the right ventricular coil to theindifferent electrode. Alternatively, the atrial evoked response fromunipolar pacing may be sensed with an attenuation means be sensingbetween the atrial tip electrode to the atrial ring electrode, theatrial tip electrode to the superior vena cava coil, the atrial tipelectrode to the ventricular tip electrode, the atrial tip electrode tothe ventricular ring electrode, the atrial tip electrode to the rightventricular coil, the atrial tip electrode to the can, the atrial tipelectrode to the indifferent electrode, the atrial ring electrode to thecan, the superior vena cava coil to the can ventricular tip electrode tothe can, the ventricular ring electrode to the can, the rightventricular coil to the can, and the can to the indifferent electrode.

When bipolar pacing in the atrium, the atrial evoked response may besensed without the need for an attenuation means by sensing between thesuperior vena cava coil to the ventricular tip electrode, the superiorvena cava coil to the ventricular ring electrode, the superior vena cavacoil to the right ventricular coil, the superior vena cava coil to thecan, the superior vena cava coil to the indifferent electrode, theventricular tip electrode to the ventricular ring electrode, theventricular tip electrode to the right ventricular coil, the ventriculartip electrode to the can, the ventricular tip electrode to theindifferent electrode, the ventricular ring electrode to the can, theventricular ring electrode to the indifferent electrode, the rightventricular coil to the can, and the right ventricular coil to theindifferent electrode. Alternatively, the atrial evoked response frombipolar pacing may be sensed with an attenuation means by sensingbetween the atrial tip electrode to the atrial ring electrode, theatrial tip electrode to the superior vena cava coil, the atrial tipelectrode to the ventricular tip electrode, the atrial tip electrode tothe ventricular ring electrode, the atrial tip electrode to the rightventricular coil, the atrial tip electrode to the can, the atrial tipelectrode to the indifferent electrode, the atrial ring electrode to thesuperior vena cava coil, the atrial ring electrode to the ventriculartip electrode, the atrial ring electrode to the ventricular ringelectrode, the atrial ring electrode to the right ventricular coil, theatrial ring electrode to the can, the atrial ring electrode to theindifferent electrode, and the can to the indifferent electrode.

FIGS. 4-6 illustrate alternative embodiments of a portion of thepacing/sensing circuit 22 detailing the electrode elements of cardiacpacer 10 for exemplary purposes. FIG. 4 shows a dedicated atrial evokedresponse amplifier 40 electrically coupled between the atrial ringelectrode 30 and the ventricular tip electrode 32. FIG. 5 shows analternate embodiment of a portion of the pacing and sensing circuit 22,wherein the dedicated atrial evoked response amplifier 40 iselectrically coupled between the atrial ring electrode 30 and theindifferent electrode 24 and a dedicated ventricular evoked responseamplifier 44 is electrically coupled between the ventricular ringelectrode 34 and the ventricular tip electrode 32. FIG. 6 shows analternate embodiment of a portion of the pacing and sensing circuit 22,wherein the dedicated atrial evoked response amplifier 40 iselectrically coupled between the indifferent electrode 24 and theventricular tip electrode 32 and a dedicated ventricular evoked responseamplifier 44 is electrically coupled between the ventricular ringelectrode 34 and the ventricular tip electrode 32.

The inventors have found that the affects of the pacing afterpotentialson the sensed evoked response during autocapture detection may beavoided when the pacing electrodes and the sensing electrodes areindependent of one another. Therefore, the use of independent pacing andsensing electrodes in certain configurations eliminates the need for anattenuation means for these configurations. Additionally, the pacingcircuit of the present invention may be utilized when sensing an evokedresponse in accordance with the above configurations or when utilizingan electrode for both pacing and sensing in combination with anattenuation means.

Referring to FIG. 7, a portion of the exemplary embodiment of the pacingand sensing circuit 22 shown in FIG. 4 is illustrated in greater detail.Those skilled in the art will appreciate that pacing/sensing circuit 22may be modified slightly to achieve any of the above identified sensingconfigurations for atrial evoked response or any of the above identifiedsensing configurations for ventricular evoked response. Thus, thedescription of the pacing/sensing circuit as shown in FIG. 4 should notbe construed as limiting. As will be explained below, the improvedcircuit 22 is capable of quickly attenuating any polarization voltagesor “afterpotential” which result due to the application of stimuluspulses to the heart 26. By attenuating the polarization voltages or“afterpotential” in this fashion, the improved circuit 22 facilitatesthe task of capture verification in that the presence or absence ofevoked responses may be readily determined without the masking caused byafterpotential. Capture verification advantageously allows the pacemaker10 to automatically adjust the pacing output parameters so as tominimize power consumption while assuring therapeutic efficacy.

In the exemplary embodiment shown in FIG. 7, the circuit 22 includes apower supply or battery 46, a first switch (S1) 48, a second switch (S2)50, a third switch (S3) 52, a pacing charge storage capacitor (C1) 54,and an afterpotential reduction capacitor/coupling capacitor (C2) 56,all of which are cooperatively operable under the direction of acontroller of known suitable construction. The power supply or battery46 is preferably the battery provided to power the pacemaker 10 and maycomprise any number of commercially available batteries suitable forpacing applications. The switches 48-52 are preferably carried out viaany number of conventionally available microprocessor-directedsemiconductor integrated circuit switching means. The pacing chargestorage capacitor 54 may also comprise any number of conventionalstorage capacitors, but is preferably provided with a capacitance in therange of 10-30 microfarads so as to develop a sufficient pacing chargefor stimulating the heart 26. The primary function of the couplingcapacitor 56 is to quickly attenuate the polarization voltage or“afterpotential” which result from pacing and additionally block any DCsignals from reaching the heart 26 during pacing. The coupling capacitor56 has a capacitance in the range less than 5 microfarads, with a 2.2microfarad capacitor being preferred.

The sensing portion of the circuit 22 includes pace blanking switches 58and 60, passive filters 62 and 64, voltage reference 66, sense amplifierblanking switches 68 and 70, preamplifier 72, band pass filter 74,analog to digital converter 76 and detection comparator 78. Thecontroller is operatively coupled to the circuit 22 and controls theopening and closing of switches 58, 60, 68, and 70. Although switches58, 60, 68, and 70 are illustrated as discrete components, those skilledin the art will appreciate that they may comprise any number ofcommercially available microprocessor-directed semiconductor integratedcircuit switching means. The pace blanking switches 58 and 60 are closedindependently to detect an evoked response from the corresponding pacingelectrode, and the shortening of the pacing afterpotential by using areduced capacitance coupling capacitor allows pacing and sensing of theevoked response from the same electrodes. The intrinsic sensing channelmay also be shared for efficient system operation. By shortening thepacing afterpotential, the recharge time of the coupling capacitor 56may be reduced from a conventional time of greater than 20 millisecondsto under 10 milliseconds. This shortened time usually lapses before theonset of an evoked response. In turn, the sense amplifier blanking timemay be reduced from a conventional 30 milliseconds to under 15milliseconds with 12 milliseconds being preferred. This shortenedblanking period in conjunction with the shortening of the pacingafterpotential increases the likelihood of detecting an evoked response.

Having described the constructional features of the pacing and sensingcircuit the mode of use will next be described in greater detail. Thecontroller implements a pre-programmed sequence to control the chargingcycle, pacing cycle, and recharge cycle of the pacing output circuit.The charging cycle is characterized as having the first switch 48 in aclosed state with the second switch 50 and third switch 52 in an openstate. In this configuration, the pacing charge storage capacitor 54 maybe charged up to a predetermined pacing voltage level, such as 3 volts.After the pacing charge storage capacitor 54 has been charged up to thepredetermined pacing voltage level, the pacing cycle then operates todeliver the pacing charge from the pacing charge storage capacitor 54 tothe heart 26.

To accomplish the pacing cycle, the first switch 48 is opened and thirdswitch 52 remains opened and the second switch 50 is closed. This allowsthe voltage within the pacing charge storage capacitor 54 to bedischarged through the coupling capacitor 56 to the tip electrode 28positioned in the heart 26. The coupling capacitor 56 is less than 5microfarads. This, once again, effectively blocks any significant DCsignals from reaching the heart 26, while shortening the pacingafterpotential. Those skilled in the art will appreciate that in thoseconfigurations where the pacing is between the tip electrode and the canand sensing is between an atrial lead electrode and ventricular leadelectrode as described above, a coupling capacitor of known suitableconstruction may instead be utilized.

The recharge cycle involves keeping open the first switch 48 and openingthe second switch 50 while closing the third switch 52. This allows thecircuit 22 to passively recharge, such that the charge within the heart26 is allowed to flow back into the pacing output circuit to balanceout. During this passive recharge period, the charge on the couplingcapacitor 56 is such that the signal decays over a short period of timeand less than the required blanking period preceding detection of anyevoked response from the heart 26. This is because the evoked responsesfrom the heart 26 typically begins within 12 milliseconds from thedelivery of a stimulus pulse to the atrium and within 20 millisecondsfrom the delivery of a stimulus pulse to the ventricle, which issubstantially longer than the required recharge time. Advantageously, ithas been found that reducing the overall capacitance of the couplingcapacitor 56 quickly attenuates the polarization voltages or“afterpotentials” which result immediately following the application ofa stimulus pulse such that the evoked responses within the heart 26 willnot be masked or buried within the “afterpotential.” By eliminating theadverse affects of “afterpotential” in this fashion, the pacemaker 10can easily sense an evoked response and track the capture threshold ofthe heart 26 over time. Those skilled in the art will appreciate thatwith the continuous evaluation of an evoked response, the pacemaker 10may be automatically adjusted to maintain an optimal pacing stimuluslevel which ensures safe pacing while minimizing power consumption.

Referring now to FIG. 8, a portion of the pacing and sensing outputcircuit 22 is shown having a modified pacing circuit 80 for exemplarypurposes, wherein the circuit 80 is capable of quickly attenuatingpolarization voltages or “afterpotential” which result due to theapplication of stimulus pulses to the heart 26. By attenuating thepolarization voltages or “afterpotential” in this fashion, the improvedpacing circuit 80 of the present invention facilitates the task ofcapture verification in that the presence or absence of evoked responsesmay be readily determined without the masking caused by afterpotential.Capture verification may advantageously allow the pacemaker 10 toautomatically adjust the capture threshold so as to minimize powerconsumption while assuring therapeutic efficacy.

The pacing output circuit 80 of the present invention includes a powersupply or battery 82, a first switch 84, a second switch 86, a thirdswitch 88, a fourth switch 90, a pacing charge storage capacitor 92, afirst coupling capacitor 94, and a second coupling capacitor 96, all ofwhich are cooperatively operable under the direction of a controller. Byway of example, the improved pacing output circuit 80 is illustrated ina ventricular pacing arrangement for delivering stimulus pulses to theheart 26 via the tip electrode 32 and ring electrode 34 of theventricular pacing lead 14 shown in FIG. 1. It is to be readilyunderstood, however, that the improved pacing output circuit 80 of thepresent invention may also find application in an atrial pacingarrangement.

The power supply or battery 82 is preferably the battery provided topower the pacemaker 10 and may comprise any number of commerciallyavailable batteries suitable for pacing applications. The switches 84-90are illustrated as discrete components but are preferably carried outvia any number of commercially available microprocessor-directedsemiconductor integrated circuit switching means. The pacing chargestorage capacitor 92 may also comprise any number of commerciallyavailable storage capacitors, but is preferably provided with acapacitance in the range greater than 10 microfarads so as to develop asufficient pacing charge for stimulating the heart 26.

One function of the second coupling capacitor 96 is to block DC signalsfrom reaching the heart 26 during pacing. In order to minimize thepacing pulse droop the second coupling capacitor 96 should have asufficiently large capacitance, for example, greater than 10microfarads. In an important aspect of the present invention, the firstcoupling capacitor 94 is advantageously provided having a capacitancepreferably less than 5 microfarads and substantially smaller than thatof the second coupling capacitor 96. As will be described in greaterdetail below, the first coupling capacitor 94 may be selectivelyoperable, via the fourth switch 90, so as to selectively reduce theeffective capacitance of the second coupling capacitor 96, therebyquickly attenuating the polarization voltage or “afterpotential” whichresult from pacing.

Having described the constructional features of the modified pacingcircuit 80, the operation of the pacing output circuit 80 will now bedescribed. During a normal pacing mode, the pacing output circuit 80engages in a charging cycle, a pacing cycle, and a recharge cycle. Thecharging cycle is characterized as having the first switch 84 in aclosed state with the second and third switches 86-90 in an open state.In this configuration, the pacing charge storage capacitor 92 may becharged up to a predetermined pacing voltage level, such as 3 volts.After the pacing charge storage capacitor 92 has been charged up to thepredetermined pacing voltage level, the pacing cycle then operates todeliver the pacing charge from the pacing charge storage capacitor 92 tothe heart 26. To accomplish this pacing cycle, the first switch 84 andthird switch 88 are in the open state and the second switch 86 andfourth switch 90 may be in the closed state. This allows the voltagewithin the pacing charge storage capacitor 92 to be discharged throughthe second coupling capacitor 96 to the tip electrode 32 of thepacemaker 10. Maintaining the fourth switch 90 in a closed stateeffectively bypasses the first coupling capacitor 94 such that thesecond coupling capacitor 96 is at its full capacitance level ofapproximately greater than 10 microfarads. This, once again, effectivelyblocks any DC signals from reaching the heart 26. In another alternatepreferred embodiment, during the normal pacing mode, the fourth switch90 may be open so long as the pacing threshold does not exceed apredetermined limit. In this manner detection of an evoked response(autocapture) may be enhanced during the normal pacing mode. During theautothreshold pacing mode, the fourth switch 90 is always in the openstate and is closed for normal pacing.

The recharge cycle during normal pacing involves having the first switch84 and the second switch 86 in the open state, while having the thirdswitch 88 in the closed state. This allows the circuit 80 to passivelyrecharge, such that the charge within the heart 26 is allowed to flowback into the circuit 80 to balance out. As noted above, during thispassive recharge period, the charge on the second coupling capacitor 96may be such that the afterpotential signal exponentially decays over arelatively long period of time lasting up to 100 milliseconds. Thislarge “afterpotential” signal unwontedly masks out any evoked responsefrom the heart 26. This is because the evoked responses from the heart26 typically occur within 20 milliseconds from the delivery of thestimulus pulse to the ventricle and are substantially smaller inmagnitude than the large “afterpotential” which would develop within thesecond coupling capacitor 96, were it not for the present invention.

In one embodiment of the present invention, it is an important aspect ofthe present invention that the polarization voltages or “afterpotential”which result from pacing quickly attenuate. This is achieved by havingfourth switch 90 in the open state such that the first couplingcapacitor 94 and second coupling capacitor 96 are connected in series.The series coupling of the first coupling capacitor 94 and secondcoupling capacitor 96 causes the overall capacitance to approximate thelower capacitance, or in other words, the capacitance of the firstcoupling capacitor 94. In a preferred embodiment, the first couplingcapacitor 94 may be provided having a capacitance in the range of 1-2microfarads such that, for a brief moment, the overall capacitancebetween the afterpotential reduction capacitor 94 and coupling capacitor96 is approximately 1-2 microfarads. Advantageously, it has been foundthat reducing the effective capacitance of the second coupling capacitor96 quickly attenuates the polarization voltages or “afterpotential”which result immediately following the application of a stimulus pulsesuch that the evoked responses within the heart 26 will not be masked orburied within the “afterpotential.” By eliminating the adverse affectsof “afterpotential” in this fashion, the pacemaker 10 can easilydetermine and track the capture threshold of the heart 26 over time.Those skilled in the art will appreciate that with the continuousknowledge of the capture and pacing threshold in hand, the pacemaker 10may be automatically adjusted to maintain an optimal pacing stimuluslevel which ensures safe pacing while minimizing power consumption.

Referring next to FIGS. 9 and 10, the resulting pacing waveforms 150 and152 detected with the tip and ring of a pacing lead, for theconventional pacing circuit and the pacing circuit of FIG. 9respectively, are shown for comparison. By electrical analysis theory,familiar to those skilled in the art, the pacing afterpotential signaldecay characteristics are determined by the time constant formed by theproduct of the coupling capacitor (blocking) and the load (a combinationof the impedance of the lead body, electrode/tissue interface, andmyocardium). When the capacitance of the coupling capacitor is reduced,the afterpotential has a larger initial amplitude but dissipates faster(compare afterpotential amplitudes 154 and 156 for the respective pacingafterpotential waveforms 150 and 152). The blanking period 158 beforesensing for the conventional capacitor is greater than the requiredblanking period 160 when utilizing a 1 microfarad coupling capacitor(see FIGS. 9 and 10 for comparison). Also, the recharge time 162 whenutilizing a conventional coupling capacitor is significantly longer thanthe required recharge time 164 required for the 1 microfarad capacitor.Further, the recharge time 162 overlaps into sensing period 166 for theconventional capacitor, whereas the recharge time 164 terminates priorto the beginning of the sensing period 168 for the 1 microfaradcapacitor. Hence, when the coupling capacitance is sufficiently small,for example, less than 5 microfarads, the pacing afterpotential willsettle to baseline at a faster rate and before the onset of the evokedresponse, thereby making detection of the evoked response feasible.

Those skilled in the art will appreciate that as the couplingcapacitance decreases, the pacing pulse seen by the heart will bear alarger droop and the threshold voltage that evokes a response increases.Thus, if a small coupling capacitance is utilized during a determinationof the threshold, the determined threshold will be greater than theactual threshold required during normal pacing (assuming that aconventional coupling capacitance is utilized during normal pacing),thereby increasing the pacing safety margin.

Referring next to FIGS. 11 and 12, a recorded signal sensed between theatrial tip electrode 28 and the atrial ring electrode 30 resulting froma paced stimulus between the atrial tip electrode 28 and the atrial ringelectrode 30 is shown, wherein a conventional coupling capacitor wasutilized in the pacing and sensing circuit 22. FIG. 11 illustrates aresulting output or signal 178 and corresponding surfaceelectrocardiogram (ECG) signal 180, wherein the pacing output voltage isbelow the known threshold. FIG. 12 illustrates a resulting signal 182and corresponding ECG signal 184, wherein the pacing output voltage isabove the known threshold. Those skilled in the art will appreciate thatthe intra cardiac signals 178 and 182 are overwhelmed with pacingafterpotential and, thus, the evoked response and non-captured artifactsduring capture and non-capture respectively are not easilydistinguishable within 100 milliseconds after pacing.

FIGS. 13 and 14 show recorded signals sensed between the atrial tipelectrode 28 and the atrial ring electrode 30 resulting from a pacedstimulus between the atrial tip electrode 28 and the atrial ringelectrode 30 received when implementing a 2 microfarad couplingcapacitor having an 8 millisecond recharge time and a blanking time of10 milliseconds. FIG. 13 illustrates a resulting output or signal 186and corresponding surface electrocardiogram (ECG) signal 188, whereinthe pacing output voltage is below the known threshold. FIG. 14illustrates a resulting signal 190 and corresponding ECG signal 192,wherein the pacing output voltage is above the known threshold. Theevoked response and non-captured artifacts are readily distinguishableduring capture and non-capture for signals 186 and 190. Withoutlimitation, a conventional peak detector may be adapted for detectingthe peaks in the recorded signal received after pacing while using a 1microfarad coupling capacitor having a 8 millisecond recharge time.

FIGS. 15 and 16 show recorded signals sensed between the atrial ringelectrode 30 and the indifferent electrode 24 resulting from a pacedstimulus between the atrial ring electrode 30 and the can 18. Therecorded signals were received while implementing a 2 microfaradcoupling capacitor having a 10 millisecond recharge time and a blankingtime of 12 milliseconds. FIG. 15 illustrates a resulting output orsignal 194 and corresponding surface electrocardiogram (ECG) signal 196,wherein the pacing output voltage is below the known threshold. FIG. 16illustrates a resulting signal 198 and corresponding ECG signal 200,wherein the pacing output voltage is above the known threshold. Theevoked response and non-captured artifacts are readily distinguishableduring capture and non-capture for signals 194 and 198. As best seen inFIG. 17, the evoked response is readily distinguishable from outputassociated with polarization.

FIGS. 17 and 18 show recorded signals sensed between the atrial ringelectrode 30 and the ventricular tip electrode 32 resulting from a pacedstimulus between the atrial ring electrode 30 and the can 18. Therecorded signals were received while implementing a 2 microfaradcoupling capacitor having a 10 millisecond recharge time and a blankingtime of 12 milliseconds. FIG. 17 illustrates a resulting output orsignal 202 and corresponding surface electrocardiogram (ECG) signal 204,wherein the pacing output voltage is below the known threshold. FIG. 18illustrates a resulting signal 206 and corresponding ECG signal 208,wherein the pacing output voltage is above the known threshold. Theevoked response and non-captured artifacts are readily distinguishableduring capture and non-capture for signals 202 and 206. As discussedabove, those skilled in the art will appreciate that noise is lesslikely to affect the recorded signal sensed between the atrial ringelectrode 30 and ventricular tip electrode 32 and further the sensingconfiguration may also be utilized to detect a ventricular evokedresponse.

FIG. 19 illustrates the resulting waveforms 200, 202, and 204 from pacedstimuli which were received when implementing a 2 microfarad couplingcapacitor having an 8 millisecond recharge time and a blanking time of10 milliseconds. The waveform 200 was detected between the superior venacava coil electrode and an indifferent electrode 26 positioned on thecan 18. Waveform 202 was detected between the ventricular coil electrodeand an atrial ring electrode. Waveform 204 was detected from aconventional surface electrocardiogram. The portion of waveforms 200 and202 indicated as non-capture are the result of pacing stimulus belowthreshold. Those skilled in the art will appreciate that the evokedresponse and non-captured artifacts are readily distinguishable duringcapture and non-capture for signals 200 and 202. Without limitation, aconventional peak detector may be adapted for detecting the peaks in therecorded signal received after pacing while using a 1-15 microfaradcoupling capacitor having an 8 millisecond recharge time.

This invention has been described herein in considerable detail in orderto comply with the Patent Statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to the equipment details and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

1. A method for operating a cardiac rhythm management device,comprising: switchably connecting at least two electrodes disposed in anatrium or a ventricle to the inputs of a sense amplifier; delivering anelectrical stimulus to the atrium or ventricle through a couplingcapacitance wherein the electrical stimulus is delivered between atleast one of the electrodes connected to the sense amplifier and ahousing of the device; attenuating afterpotentials resulting from thedelivered electrical stimulus by reducing the coupling capacitance; and,sensing an evoked response to the electrical stimulus from the output ofthe sense amplifier.
 2. The method of claim 1 wherein the couplingcapacitance is reduced by switchably connecting a capacitor to anelectrode connected to the sense amplifier.
 3. The method of claim 1wherein the coupling capacitance is reduced by switchably connecting afirst coupling capacitor in series with a second coupling capacitor. 4.The method of claim 1 wherein the electrical stimulus is deliveredbetween an atrial tip electrode and a housing of the device.
 5. Themethod of claim 1 wherein the electrical stimulus is delivered between aventricular tip electrode and a housing of the device.
 6. The method ofclaim 1 wherein the evoked response is sensed between an atrial ringelectrode and a ventricular electrode.
 7. The method of claim 1 whereinthe evoked response is sensed between two ventricular electrodes.
 8. Themethod of claim 1 wherein the evoked response is sensed between anatrial ring electrode and a ventricular ring electrode.
 9. The method ofclaim 1 wherein the evoked response is sensed between an atrial ringelectrode and a housing electrode.
 10. The method of claim 1 wherein theevoked response is sensed between an atrial ring electrode and aventricular coil electrode.
 11. The method of claim 1 wherein the evokedresponse is sensed between an atrial ring electrode and a superior venacava coil electrode.
 12. The method of claim 1 wherein the evokedresponse is sensed between an atrial tip electrode and a ventricularcoil electrode.
 13. The method of claim 1 wherein the evoked response issensed between an atrial tip electrode and a ventricular tip electrode.14. The method of claim 1 wherein the evoked response is sensed betweenan atrial tip electrode and an atrial ring electrode.
 15. The method ofclaim 1 wherein the evoked response is sensed between a superior venacava coil electrode and an atrial tip electrode.
 16. The method of claim1 wherein the evoked response is sensed between a superior vena cavacoil electrode and a ventricular coil electrode.
 17. The method of claim1 wherein the evoked response is sensed between a superior vena cavacoil electrode and a ventricular tip electrode.
 18. The method of claim1 wherein the evoked response is sensed between a ventricular tipelectrode and a ventricular coil electrode.
 19. The method of claim 1wherein the evoked response is sensed between a superior vena cava coilelectrode and a ventricular ring electrode.
 20. The method of claim 1wherein the evoked response is sensed between a ventricular ringelectrode and a ventricular coil electrode.